The present invention relates to a photo-sensing device for sensing patterns on cloths in a sewing machine, especially to a normalization setting of the photo-sensing device.
Photo-sensors are widely used in industrial machines and robots. In a sewing machine disclosed in the U.S. Pat. No. 4,612,867, two photo-sensors are used to sense patterns on two cloths having the same pattern in order to sew them with their patterns matching. Each of the photo-sensors includes a light-emitting element that casts light on a cloth and a light-receiving element that senses its pattern. The assignee of the present application has filed a Japanese Patent Application No. S61-274563 (now Published Unexamined Japanese Patent Application No. S63-127785) in which one photo-sensor is used in a pattern-matching sewing machine in two ways: for sensing patterns, and for detecting a cloth-edge. When a cloth ends at the light-casting or detecting spot, the light is reflected by a high-reflectance plate instead of by the low-reflectance cloth surface.
In the latter sewing machine, the photo-sensor system treats two kinds of photo-signals: a pattern signal, and a cloth-edge signal. A problem with this pattern-matching sewing machine arises when the light-emitting element or the light-receiving element degenerates during use. The photosensor system is calibrated at its creation so that the strength of light emitted from the light-emitting element is at an appropriate level and, as shown in FIG. 5, the pattern signals generated by the light-receiving element fall between V1 and V2, and the cloth-edge signal is around V3, all of which are within output voltage levels corresponding to the normal sensitivity range of the light-receiving element. The output voltage of the light-receiving element is almost proportional to the strength of the received light in the normal sensitivity range, but saturates over a certain upper limit because the output voltage cannot exceed the source voltage Vcc of the sensor. Since V1, V2 and V3 are all generated by one light-receiving element, the pattern signal and the cloth-edge signal change correspondingly to each other.
When V3 decreases, for example, due to aging or thermal drift of the light-emitting element or the light-receiving element, or due to variations in the efficiency of individual sensors, or due to a mis-alignment of the optical path, V1 and V2 also decrease, their voltage latitude becomes narrower, and the pattern sensing becomes difficult. In this case, an amplification of the output voltage of the light-receiving element will not work well because noise signals (such as from ambient light) would also be amplified. When, on the contrary, V3 increases, V2 and V3 come closer because V3 approaches its upper limit, so the pattern signal and the cloth-edge signal may be confused.
In Published Unexamined Japanese Patent Application No. S60-85385, a controller for a photo-sensor of a sewing machine is disclosed. The photo-sensor is used to detect cloth edges and overlapping areas of cloths between a light-emitting element and a light-receiving element. The sensor controller adjusts the strength of light emitted by the light-emitting element according to the thickness of the cloth to properly detect the cloth-edge and the overlapping area. This sensor controller, however, only changes the strength of the emitted light; it does not address the normal operable sensitivity range or the effective sensitivity range of the light-receiving element. Thus, the sensor controller cannot compensate for changes in the efficiency of the light-emitting element or the light-receiving element.